The Global Positioning System, usually called GPS, is the only fully-functional satellite navigation system. A constellation of more than two dozen GPS satellites broadcasts precise timing signals by radio to GPS receivers, allowing them to accurately determine their location (longitude, latitude, and altitude) in any weather, day or night, anywhere on Earth.GPS has become a vital global utility, indispensable for modern navigation on land, sea, and air around the world, as well as an important tool for map-making and land surveying. GPS also provides an extremely precise time reference, required for telecommunications and some scientific research, including the study of
The United States Department of Defense developed the system, officially named NAVSTARGPS(Navigation Signal Timing and Ranging GPS), and launched the first experimental satellite in 1978. The satellite constellation is managed by the 50th Space Wing. Although the cost of maintaining the system is approximately US$400 million per year, including the replacement of aging satellites, GPS is available for free use in civilian applications as a public good.
HistoryThe first satellite navigation system, Transit, used by the United States Navy, was first successfully tested in 1960. Using a constellation of five satellites, it could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the Timation satellite which proved the ability to place accurate clocks in space, a technology the GPS system relies upon. In the 1970s, the ground-based Omega Navigation System, based on signal phase comparison, became the first world-wide radio navigation system. The most recent launch was in September 2005. The oldest GPS satellite still in operation was launched in February 1989.
MilitaryGPS allows accurate targeting of various militaryweapons including cruise missiles and precision-guided munitions, as well as improved command andcontrol of forces through improved locationalawareness. The satellites also carry nucleardetonation detectors, which form a major portion ofthe United States Nuclear Detonation DetectionSystem. Civilian GPS receivers are required to havelimits on the velocities and altitudes at which theywill report coordinates; this is to prevent them frombeing used to create improvised missiles.
NavigationGPS is used by people around the world as anavigation aid in cars, airplanes, and ships. Hand-heldGPS receivers can be used by mountain climbers andhikers. Glider pilots use the logged signal to verifytheir arrival at turn points in competitions. Low costGPS receivers are often combined with PDAs, cellphones, car computers, or vehicle tracking systems.Examples of GPS-based services are MapQuest Mobileand TomTom digital maps. The system can be used toautomate harvesters, mine trucks, and other vehicles.GPS equipment for the visually impaired is available.
GPS satellites broadcast three different types of datain the primary navigation signals. The first isthealmanac which sends coarse time information withsecond precision along with status information aboutthe satellites. The second is the ephemeris, whichcontains orbital information that allows the receiverto calculate the position of the satellite at any point intime. These bits of data are folded into the 37,500bit Navigation Message, or NM, which takes 12.5minutes to send at 50 Hz.
Calculating positionsGPS allows receivers to accurately calculate theirdistance from the GPS satellites. The receivers do thisby measuring the time delay between when thesatellite sent the signal and the local time when thesignal was received. This delay, multiplied by thespeed of light, gives the distance to that satellite. Thereceiver also calculates the position of the satellitebased on information periodically sent in the samesignal. By comparing the two, position and range, thereceiver can discover its own location.
AccuracyBest Case:The position calculated by a GPS receiver relies onthree accurate measurements: the current time, theposition of the satellite, and the time delay for thesignal. The overall accuracy of the system is generallybased on the accuracy of the position and delay.
Atmospheric effectsOne of the biggest problems for GPS accuracy is thatchanging atmospheric conditions change the speed ofthe GPS signals unpredictably as they pass throughthe ionosphere. The effect is minimized when thesatellite is directly overhead and becomes greatertoward the horizon, since the satellite signals musttravel through the greater "thickness" of theionosphere as the angle increases. Once the receiversrough location is known, an internal mathematicalmodel can be used to estimate and correct for theerror.
SatellitesAs of August 2006 the GPS system used a satelliteconstellation of 29 active Block II/IIA/IIR/IIR-M satellites(for the global coverage 24 is enough) in intermediatecircular orbits. The constellation includes three sparesatellites in orbit, in case of any failure. Each satellitecircles the Earth twice each day at an altitude of 20,200kilometers (12,600 miles). The orbits are aligned so at leastfour satellites are always within line of sight from almostany place on Earth. There are four active satellites in eachof six orbital planes. Each orbit is inclined 55 degrees fromthe equatorial plane, and the right ascension of theascending nodes is separated by sixty degrees.
Frequencies usedSeveral frequencies make up the GPS electromagneticspectrum:L1 (1575.42 MHz):Carries a publicly usable coarse-acquisition (C/A) code aswell as an encrypted precision P(Y) code.L2 (1227.60 MHz):Usually carries only the P(Y) code, but will also carry asecond C/A code on the Block III-R satellites.L3 (1381.05 MHz):Carries the signal for the GPS constellations alternativerole of detecting missile/rocket launches (supplementingDefense Support Program satellites), nuclear detonations,and other high-energy infrared events.
GPS trackingA GPS tracking system uses GPS to determine thelocation of a vehicle, person, or pet and to record theposition at regular intervals in order to create a trackfile or log of activities. The recorded data can bestored within the tracking unit, or it may betransmitted to a central location, or Internet-connected computer, using a cellular modem, 2-wayradio, or satellite. This allows the data to be reportedin real-time, using either web browser based tools orcustomized software.
AwardsTwo GPS developers have received the National Academyof Engineering Charles Stark Draper prize year 2003:Ivan Getting, emeritus president of The AerospaceCorporation and engineer at the Massachusetts Instituteof Technology, established the basis for GPS, improving onthe World War II land-based radio system called LORAN(Long-range Radio Aid to Navigation).Bradford Parkinson, teacher of aeronautics andastronautics at Stanford University, developed the system.One GPS developer, Roger L. Easton, received theNational Medal of Technology on February 13, 2006 at theWhite House.